Battery charging

Battery charging

To fully exploit the advantages of electric-powered flight, you will need a “peak-detect” battery charger. Basic chargers that operate simply from a timing device can easily cause over- or under-charging. The peak-detect charger senses when the battery pack reaches full capacity and shuts itself off automatically (some revert to a trickle or sustaining charge level). So do yourself a favor and invest in a peakdetect charger right from the start; prices begin at slightly more than $100. My favorite, because it is inexpensive, easy to operate and reliable, is the AstroFlight 110D. It can handle up to 18 Ni-Cd or NiMH cells at up to 5 amps charge current. It operates from your 12V car battery (or a battery of comparable size). Note: do not attempt to use your little 12V field kit (motorcycletype) battery. It has only a fraction of the current capacity of a car battery, and the charge will be quickly depleted after only a few flights. With the diminishing capacity, the fast-charge period will take longer and longer, and this is frustrating.

All new Ni-Cd and NiMH batteries should initially be charged at the overnight rate. A 1200mAh battery pack will need 120mA (1200 divided by 10) of charge current. My favorite overnight charger is the Ace R/C (now Ace Hobby Distributors) Digital Dual Vari-Charger (DDVC), which has two adjustable outputs from 0 to 500mA and operates from either a 12V DC source or from 115V AC household power. (For more details on this DDVC, see my review in the May 2000 issue of Model Airplane News.)

CHARGE RATES

There are three basic types of battery charging rates: “fast,” “overnight” and “trickle.” The fast-charge rate can usually bring a battery up to full charge in less than 30 minutes. This fast rate is used at the flying field and allows you to make many flights during the course of a day. The fast-charge rate is generally between two and three times the battery capacity in mAh, depending on the type of battery. Batteries used for fast charging in the field are so specified by their manufacturers/distributors.

Overnight charge rate (also referred to as “slow” charge rate) is calculated by taking the battery capacity in mAh and dividing by 10. A full overnight charge may take from 10 to 24 hours to complete and helps to condition or equalize the various cells that make up a battery pack. Overnight is also the charge rate of choice when using standard battery cells.

Trickle-charge rate is a very low “maintenance”-level charge that’s derived by dividing the battery capacity in mAh by 50. This charge rate can be left on the battery indefinitely. Tricklecharging is rarely done with drive-system power packs; it is more common for radio-system batteries.

SELECTING FAST CHARGE CURRENT

For Ni-Cd batteries, the fast-charge current should be determined by taking the rated capacity (in mAh or amp hours) and multiplying that by three (abbreviated as 3C). This 3C rate is probably the best all-around fast-charge rate for the beginner. Again, let’s take a battery rated at 1200mAh (1.2 amp hours). When you multiply 1.2 amp hours by 3, you get 3.6 amps, so set your peak-detect charger for 3.6 amps. If the pack is fully depleted, it will take approximately 20 minutes to reach full charge.

Setting the fast charge rate is a very important step. Unless you own one of the very new, fully automatic battery chargers, you will have to choose the appropriate correct charge current. Set it too low, and it will take too long to charge your batteries. Set it too high, and you will seriously overheat your batteries and possibly ruin them. As you get more experienced, you will find that certain higher capacity Ni-Cd batteries can take more than the 3C fast-charge rate, but to begin with, it is better to be cautious.

NiMH batteries must be treated a little more conservatively. For these cells, you should use only twice the rated capacity, or 2C. A 1200mAh NiMH battery would be charged at 2.4 amps (1.2×2). To reach a full charge will take approximately 30 minutes.

The lithium-metal batteries are usually rated at 800mAh. Chargers for this type of cell are set at about 80mA current (although several use a higher rate) and will automatically cut off when the voltage under charge reaches 3.4 volts per cell. Generally, these specialty chargers can be set to charge one, two, or three lithium-metal cells. A good charger for this purpose is the Magellan, manufactured by Magellan Technologies Inc. (reviewed in the January 2000 issue of RC MicroFlight). Another excellent charger is the JMP, designed by J.M. Piednoire of France and sold by various European distributors, including Chris Stewart of RCS Technik in Great Britain. There are several other types of lithium batteries now on the market; some are rechargeable, some are not.

CHARGING ROUTINE

I generally overnight-charge my battery packs at the C/10 (capacity divided by 10) rate a day or two before I plan to go flying. When I get to the field, I make my first flight of the day on that overnight charge. That usually does not produce a strong flight, but it does help to condition the battery. After that, I fast-charge the batteries using the peak-detect charger; this produces stronger results for the remaining flights of the day. When I return home, I put the batteries on overnight charge before storing them, but some modelers store their packs discharged; neither method will harm the batteries.

MAINTENANCE TIPS

* Partial charging. When you fast-charge a battery pack on a peak-detect charger, the battery will be fully charged automatically; the full charge is what shuts the charger off. For overnight charging, the charger should be left on for at least 10 hours; longer won’t hurt! Some modelers mistakenly believe that if only a small amount of charge is used up, then only a short recharge is necessary. This is definitely not the case. The battery’s chemistry is designed to receive a full charge. Never partially charge a battery. This is especially true for RC system batteries.

* Cooling your batteries. If a battery is fast-charged at the proper current level on a peak-detect charger, it should not get more than moderately warm using the 20 and 3C charge rates. When powering a model in flight, however, current levels can average from 10 to 30 amps and can be as high as 50 amps or more on some specialty planes. At these high currents, the battery can build up substantial heat during a flight. Sometimes, it can even become too hot to handle! You must cool a battery pack down before you attempt to recharge it. If the pack is hot when you start charging, you can easily damage or even ruin it. A common mistake among modelers is to use ice or dry ice in a cooler to quickly reduce battery temperature, but this causes very uneven temperatures throughout the pack.

My solution, thanks to Ric Vaughn (an AMA/NEAC Electric Nationals first-place winner in 1999), is to use a RadioShack 12V electric fan (catalog no. 273-243) and insert it into one end of a 12-inch-long piece of 3-inch-diameter PVC tubing. This fan draws only about 100mA of power, so it can be connected all day to your car battery without a problem. You insert the hot battery pack into the open end of this tube and simply let the air from the fan flow over it. In about 5 minutes, the battery pack will be cool enough to charge. If you do this every time you recharge your battery packs, they should last a long time.

Cycling. You’ll hear the word “cycling” when it comes to RC system batteries. This refers to the technique of discharging a battery pack all the way down to its minimum charge (usually 1 volt per cell) then recharging it. Battery capacity can be measured (in mAh) during the discharge. This process is said to eliminate the “memory” effect, whereby the battery gets used to providing only a certain amount of its total capacity.

When used for electric-powered flight, a battery is taken down to practically minimum capacity every time. As such, you are essentially cycling these batteries every time, so nothing else needs to be done to them. You can, of course, discharge them once in a while to measure the effective capacity of a pack.

Copyright Air Age Publishing Apr 2001

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